The present invention relates to a production management system for use with an automatic production line for manufacturing automobiles or the like.
As more useful electronic devices are developed in recent years, production lines for manufacturing various products are being automatized to improve the quality of the manufactured products. Modern production lines include a plurality of assembling/machining stations combined with numerically controlled machine tools and automatic machines such as robots. These assembling/machining stations are coupled with an automatic warehouse or a material handling system including self-propelled carriages so that various workpieces such as parts to be machined and assembled can be automatically supplied to the assembling/machining stations. Such a combined system is known as an FA (Factory Automation) system, which includes a production management computer that controls the machining and assembling operation in each of the assembling/machining stations and controls or manages the flow or distribution of workpieces between the assembling/machining stations. The FA system is also employed to control automobile assembling/machining lines.
To meet diverse consumer needs, some automobile assembling/machining lines of today are each designed to assemble and machine automobiles of many different kinds based on desired combinations of types, grades, geographical regions of use, and options. It is therefore necessary to rely on a production management system for supervising information on automobiles assembled or machined on an automobile assembling/machining line, giving control information to the machine tools and automatic machines, and obtaining information with regard to how the production process is going on.
One known such production management system is disclosed in Japanese Laid-Open Patent Publication No. 61(1986)-236466. More specifically, the disclosed production management system includes a single network for transferring control information between individual pieces of production equipment and production management computers for controlling those pieces of production equipment, and data input/output devices or process control computers disposed in respective assembling/machining stations and connected to the single network.
Some production lines for manufacturing automobiles or the like are constructed such that information indicating a result of assembling or machining operation such as an assembling failure or the like at a certain assembling/machining station is transmitted to a subsequent assembling/machining station in which such an assembling failure will be removed. This system mainly serves the purpose of averaging the working times in the respective assembling/machining stations to shorten the track time of the production line.
If the control information for the pieces of production equipment in the assembling/machining stations and the result information are to be processed by the single network, the amount of information to be processed by the network is so vast that the time required to process, such information is not negligible as compared with the track time of the production line. Moreover, since the period of time required to construct the network system is considerably long, the shipment of new products produced by the production line may be delayed. In addition, any failure or fault at one location in the network tends to bring about a shutdown of the entire production line.
The conventional production lines are designed with greater emphasis on the control of operation of the automatic machines, the control of the flow of workpieces, and the supervision of the number of assembled products. When a workpiece assembling/machining failure occurs in the automatic machine at a certain assembling/machining station, the workpiece cannot be fed from that assembling/machining station to a downstream assembling/machining station unless the workpiece assembling/machining failure is removed in the previous assembling/machining station. Under worst conditions, the workpieces cannot be fed from station to station along the production line.
The above drawback may be eliminated by a production management system in which means for determining the results of operation at the respective assembling/machining stations are associated with the drive shafts, respectively, of the automatic machines in the respective stations, and output signals from such determining means are sent to downstream assembling/machining stations attended by workers. Should any one of the automatic machines produce an assembling/machining failure, then the number of the drive shaft of that automatic machine is transmitted by way of the output signal of the determining means. The number is then displayed on display units in the downstream assembling/machining stations, or lamps located in the downstream assembling/machining stations and corresponding to the drive axis of the failing automatic machine are energized, thus letting the workers know the assembling/machining failure. The production management system of such a construction however consumes a long period of time before it can identify the location of the failure on the workpiece, and as a result the delivery of the workpieces along the production line needs to be stopped.
There is a demand for a system to be used in combination with the production line, described above, for effectively preventing a defective product from being manufactured. For example, Japanese Laid-Open Patent Publication No. 55(1980)-18349 discloses one such concept for preventing a defective product from being fabricated. The disclosed concept is directed to a method of determining whether a threaded component has been properly tightened or not by a nut runner. According to the disclosed method, the time required to tighten a bolt with the nut runner is measured, and it is determined whether the measured time is within a predetermined preset time or not. If the bolt is properly tightened, then "OK" is displayed on a display unit, and if the bolt is improperly tightened, then "NG" is displayed on the display unit. This method can reliably discover a bolt tightening failure, and hence can prevent a defective product with an improperly tightened bolt from being produced. When "NG" is displayed, such NG information is fed back to control the nut runner so that any subsequent operation of the nut runner is stopped. In this manner, no defective products will be manufactured.
According to the concept disclosed in Japanese Laid-Open Patent Publication No. 55(1980)-18349, however, a failure of an automatic machine such as a nut runner is detected only when a failure such as a bolt tightening failure is caused. Therefore, the production line must be shut off at the time the nut runner failure is detected. It is impossible to produce products within a predetermined track time.